This invention relates to a rear projection-type or direct viewing-type image display device and a Fresnel lens sheet used for both of these types of image display devices.
In a rear projection-type image display device for enlarging and projecting a small size image on a screen, reducing the volume of the device by reducing its depth, is generally desirable. If the depth dimension of the image display device and screen (herein often referred to as a “set”) is reduced, the incident angle of light becomes wider on the screen, especially at the periphery of the screen. For this reason, the image display device's efficiency is decreased at the periphery of the screen. This is usually the result of reflection losses, and causes the screen image to be darker in the periphery. One conventional technology for coping with this reduction in display efficiency is disclosed in JP 2004-170862 A. That document discloses that a light entrance plane of a Fresnel lens sheet (image source side) as one constituent of a screen provided with a prism having a refraction plane and a total reflection plane. The light of large incident angle is refracted by the refraction plane onto the total reflection plane, and exits to the image observation side. It is also known to reduce the depth dimension of an image display device of the direct view type by using a plurality of cold cathode florescent lamps on a light entrance side that is the rear of the image source (for example, a liquid crystal panel) as disclosed in JP 2004-273396 A.